The present invention relates to a transparent gel and a method of manufacturing the same gel. More particularly, the invention is concerned with a porous poly(vinyl alcohol) gel of high mechanical strengths, high water contents, and excellent transparency, and with a method of producing the gel.
A hydrogel is defined as the gel which contains water but is not soluble in water. Such hydrogels have been well-known for a long time, but it is just recently that the hydrogel has attracted much attention because of its novel properties. Current applications of the hydrogel include biomaterials such as soft contact lens and matrix for drug sustained release, matrix for immobilizing enzymes and microorganisms, a covering medium for maintenance of low temperatures, matrix for perfume release and so on.
The starting polymers for the gel formation involve gelatin, carrageenan, alginic acid, poly (2-hydroxyethyl methacrylate), carboxylated methylstarch, hydrolyzate of acrylonitrile-grafted starch, polyacrylamide, poly(acrylic acid) salt, hydrolyzate of vinyl acetatemethyl acrylate copolymer, polyoxyethylene, poly(vinyl pyrrolidone), polystyrene sulfonate, poly(vinyl alcohol) and so on.
It is a well-known fact that the viscosity of highly concentrated aqueous solutions of poly(vinyl alcohol) (hereinafter referred to as PVA) increase with time and the solutions finally set to a gel when allowed to stand at room temperature. The resultant gel is, however, sticky and low in mechanical strength.
A variety of methods have been reported to enhance the poor mechanical strength of the hydrated PVA gel. Among them are chemical crosslinking of PVA with aldehydes such as formaldehyde and glutaraldehyde, PVA crosslinking by irradiation with radiations such as gamma rays, electron beams, and ultraviolet light, PVA crosslinking through coordinate bonding with metal ions such as Ti, Cu, and Co, and PVA crosslinking with the use of boric acid, borax, and Congo Red. However, these proposed methods are not successful in obtaining the hydrated PVA gel which has both the high mechanical strength and the high water content. In other words, an increase in mechanical strength results in low water content, whereas a high water content leads to poor mechanical properties of the PVA gel.
In order to improve the mechanical properties of the gel without using any additives, but keeping the water content high, it was proposed to defreeze at room temperature within a short time period the highly concentrated aqueous PVA solution which had been frozen below the freezing point in a short time period (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 52296/1975). The PVA gel obtained by this method is not, however, satisfactory in the mechanical strength and becomes largely swollen upon immersion in plenty of water.
For gel formation another method was proposed, in which a frozen aqueous solution of PVA was subjected to partial freeze-drying under vacuum without defreezing (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 130543/1982). More in detail, this preparation method consists of casting a concentrated aqueous solution of PVA with a degree of saponification of not less than 95% by mole and a viscosity-average degree of polymerization of not less than 1,500 into a mold, subsequent freezing the solution at temperatures lower than -6.degree. C., and partial freeze-drying the frozen mixture under vacuum without defreezing. This method has a disadvantage in that it needs the vacuum freeze-drying process.
Also, a new method of preparing a PVA gel of high mechanical strengths was reported in Journal of Chemical Society Japan (Nippon Kagaku Kaishi), No. 9, p.1254, 1983. The gel was prepared in this method by repetition of freezing and defreezing of a concentrated aqueous PVA solution. The resulting PVA hydrogel had high elasticity, similar to the gel obtained by freezing and subsequent partial freeze-drying under vacuum.
All of these gels with high elasticity and high water contents are not colored but opaque and translucent.